1. FIELD OF THE INVENTION
The present invention relates to a piezoelectric transformer used in a backlight inverter for a liquid crystal display, an inverter for lighting a fluorescent tube, a high-voltage power supply circuit for a copying machine, and the like and, more particularly, to a compact, low-profile piezoelectric transformer of which down sizing and high reliability are required and which generates a high voltage, and a method of manufacturing the same.
2. DESCRIPTION OF THE PRIOR ART
Conventionally, in a backlight inverter for a liquid crystal display, an inverter for lighting a fluorescent tube, a high-voltage power supply circuit for a copying machine, and the like, a winding electromagnetic transformer is used as a high-voltage generating element. A piezoelectric transformer has attracted attention due to requirements such as reduction in the generated electromagnetic noise, power consumption, height, and the like.
FIG. 1A shows a conventional symmetric Rosen tertiary stacked piezoelectric transformer. The symmetric Rosen tertiary stacked piezoelectric transformer shown in FIG. 1A is constituted by a rectangular piezoelectric element 1 having electrodes formed on its surface. Regions 101 of the piezoelectric element 1 serve as the input portions of the piezoelectric transformer. Planar input electrodes 2a, 2b, 2c, and 2d are formed on the upper and lower surfaces of the input portions 101. These input portions 101 are polarized in the direction of thickness of the piezoelectric element 1. Side electrodes 3a, 3b, 3c, and 3d electrically connect the input electrodes 2a, 2b, 2c, and 2d to internal electrodes.
A region 102 of the piezoelectric element 1 which is sandwiched by the regions 101 forms the output portion of the piezoelectric transformer. Planar output electrodes 4a and 4b are formed on the upper and lower surfaces of the region 102. This output portion is polarized in the direction of length of the piezoelectric element 1.
The piezoelectric transformer shown in FIG. 1A operates in the following manner. When a voltage is applied from external terminals 5a and 5b to the input electrodes 2a, 2b, 2c, and 2d of the input portions 101, an electric field is applied to the input portions 101 in the direction of polarization. Longitudinal vibration in the direction of length of the piezoelectric element 1 is excited by the inverse piezoelectric effect in which the element is displaced in a direction perpendicular to the direction of polarization, and the entire piezoelectric element vibrates.
In the output portion 102, mechanical strain occurs in the direction of polarization of the piezoelectric element 1. Then, due to the piezoelectric positive effect in which a potential difference occurs in the direction of polarization, a voltage having the same frequency as that of the input voltage is output from the output electrodes 4a and 4b to external terminals 6a and 6b.
In the piezoelectric transformer, the output voltage is proportional to the ratio of the static capacitance of the input portions 101 to that of the output portion 102. Accordingly, a stacked piezoelectric transformer is becoming the mainstream.
A stacked piezoelectric transformer uses platinum, palladium, a silver-palladium alloy, or the like as the material of the internal electrodes, and is formed by stacking a predetermined number of internal electrode-forming piezoelectric ceramic sheets 7, as shown in FIG. 1B. The internal electrode material has an effect of promoting sintering reaction of the ceramic material. Thus, in the stacked piezoelectric transformer, the grain size of the ceramic crystal after sintering differs between the input portions where the input electrodes are present, and the output portion where the input electrodes are not present. As a result, some portions of the obtained piezoelectric transformer element are not sufficiently sintered, and a high mechanical strength and good electrical characteristics cannot be obtained.
A stacked piezoelectric transformer in which these problems are solved is disclosed in, e.g., Japanese Unexamined Patent Publication No. 8-306984. FIGS. 2A to 2C show a stacked piezoelectric transformer shown in Japanese Unexamined Patent Publication No. 8-306984, in which FIG. 2A is a perspective view, FIG. 2B is a plan view showing piezoelectric ceramic sheets formed with internal electrodes, and FIG. 2C is a sectional view of the element. Referring to FIGS. 2A to 2C, reference numeral 27 denotes a piezoelectric ceramic sheet formed with an internal electrode 28; 21, a piezoelectric ceramic rectangular plate; 22, an input electrode, 24, an output electrode, and 29, a floating internal electrode for controlling the reactivity of the ceramic crystal grain.
In the stacked piezoelectric transformer, a step formed due to the presence/absence of internal electrodes leads to cracking or separation. A stacked piezoelectric transformer in which this problem is solved is disclosed in, e.g., Japanese Unexamined Patent Publication No. 8-107241. FIG. 3 is a perspective view showing piezoelectric ceramic sheets used in the piezoelectric transformer element disclosed in Japanese Unexamined Patent Publication No. 8-107241 and formed with internal electrodes. Referring to FIG. 3, reference numeral 34 denotes an output electrode; 37, a piezoelectric ceramic sheet; 38, an internal electrode; and 39, a spacer for correcting the thickness of the internal electrode.
In a structure having the floating internal electrodes 29 formed in the output portion of the piezoelectric transformer like the stacked piezoelectric transformer of Japanese Unexamined Patent Publication No. 8-306984 shown in FIGS. 2A to 2C, although the reactivity of the ceramic crystal grains can be controlled, separation or void occurs near the floating internal electrodes while sintering the floating internal electrodes 29. Due to these separation and void, the electrical characteristics are degraded, and the effect of controlling the reactivity of the ceramic crystal grains cannot be easily obtained.
In a structure merely having the ceramic spacers 39 formed at portions excluding the internal electrodes like the stacked piezoelectric transformer of Japanese Unexamined Patent Publication No. 8-107241 shown in FIG. 3, the ceramic crystal grains at the output portion cannot be grown. In this case, although separation can be avoided, the electrical characteristics cannot be improved.